The new H1N1 flu virus could still mutate into a more virulent form and spark an influenza pandemic that could be expected to circle the globe up to three times, the World Health Organization said on Tuesday.

The impact of any pandemic would vary, as a virus that causes only mild illness in countries with strong health systems can become “devastating” in those with weak health systems, shortages of drugs and poorly equipped hospitals, it said.

The new virus, commonly referred to as swine flu, “appears to be more contagious than seasonal influenza” and nearly the world’s whole population lacks immunity to the new disease, the WHO said in a document issued overnight entitled “Assessing the severity of an influenza pandemic.”

“It attempts to explain different aspects of severity, not just the pathogenicity of the virus but its impact on health and social systems,” WHO spokesman Thomas Abraham told Reuters. “It is in response to questions from the public and media.”

The United Nations agency’s pandemic alert level is at 5, its second-highest level on a scale of 1 to 6, he said.

This means that the virus shows no signs of sustained person-to-person spread outside North America.